Asymmetric epoxidation of unsaturated ketones catalyzed by heterobimetallic rare earth-lithium complexes bearing phenoxy-functionalized chiral diphenylprolinolate ligand

Article abstract of DOI:10.1021/ol5020398

Four novel heterobimetallic complexes [REL₂]{[(THF)₃Li]₂(μ-Cl)} stabilized by chiral phenoxy-functionalized prolinolate (RE = Yb (1), Y (2), Sm (3), Nd (4), H₂L = (S)-2,4-di-tert-butyl-6-[[2-(hydroxydiphenylmethyl)pyrrolidin-1-yl]methyl]phenol have been synthesized and characterized. These readily available complexes are highly active in catalyzing the epoxidation of α,β-unsaturated ketones, while the enantioselectivity varies according to the ionic radii of the rare earth center. A series of chalcone derivatives were converted to chiral epoxides in 80 → 99% ee at 0 °C using TBHP as the oxidant in the presence of 10 mol % of 1.

Full text of DOI:10.1021/ol5020398

Letter
pubs.acs.org/OrgLett
Asymmetric Epoxidation of Unsaturated Ketones Catalyzed by
Heterobimetallic Rare Earth−Lithium Complexes Bearing Phenoxy-
Functionalized Chiral Diphenylprolinolate Ligand
Qinqin Qian,† Yufang Tan,† Bei Zhao,*,† Tao Feng,† Qi Shen,† and Yingming Yao*,†
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu
Lake Campus, Soochow University, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: Four novel heterobimetallic complexes [REL2]-
{[(THF)3Li]2(μ-Cl)} stabilized by chiral phenoxy-function-
alized prolinolate (RE = Yb (1), Y (2), Sm (3), Nd (4), H2L =
(S)-2,4-di-tert-butyl-6-[[2-(hydroxydiphenylmethyl)pyrrolidin-
1-yl]methyl]phenol have been synthesized and characterized.
These readily available complexes are highly active in catalyzing the epoxidation of α,β-unsaturated ketones, while the
enantioselectivity varies according to the ionic radii of the rare earth center. A series of chalcone derivatives were converted to
chiral epoxides in 80 → 99% ee at 0 °C using TBHP as the oxidant in the presence of 10 mol % of 1.
nantioselective epoxidation of olefins is of great
Eimportance in organic synthesis because the resulting
products, chiral epoxides, are the most versatile building blocks
for optically pure natural products and pharmaceuticals with
biological activities.1 In the 1980s, Sharpless developed an
elegant method to prepare enantiomers of epoxides from allylic
alcohols using substoichiometric amounts of titanium and
tartrate.2 This pioneering work plays a particularly important
role in asymmetric epoxidation.3 Ever since then, a range of
catalysts, which can be mainly divided into organocatalysts and
metal-based catalysts, have been developed for the asymmetric
epoxidation of alkenes, especially for that of electron-deficient
CC bonds in α,β-unsaturated ketones. Organocatalysts
capable of catalyzing asymmetric epoxidation mainly include
cinchona alkaloids,4 polyamino acids,5 and chiral amine-based
molecules.6 In addition, the work by Lattanzi et al. deserves
special comment, which represents the first successful example
of asymmetric epoxidation of α,β-unsaturated ketones catalyzed
by commercially available α,α-diphenyl-L-prolinol.7 In a
subsequent study, the chiral prolinol skeleton and its analogues
proved to be ideal organocatalysts for enantioselective
epoxidation of α,β-unsaturated ketones.8 However, this
approach suffers from relatively large catalyst loading (usually
20−30 mol %), long reaction time, required additive, and an
unsatisfying ee value. To circumvent these problems, metal-
based catalysts have been intensively studied. Various metal−
ligand combinations, e.g., ZnEt2/didentate [N,O] or [N,N]
ligands,9 n-BuLi and Bu2Mg/diethyl tartrate,10 Sc(OTf)3/N,N′-
dioxide,11 Fe/Mn complexes bearing proline-derived multi-
dentate N ligands,12 and Fe(OTf)2/phenanthroline ligands,13
have been reported to catalyze the enantioselective epoxidation.
Moreover, lanthanide−binol and its derivatives have also been
investigated, which gave rise to high enatioseletivities in many
cases and show the great potential of lanthanides in catalyzing
such a transformation.14
Heterobimetallic complexes incorporating both rare earth
(RE) and alkali metals are reported to display outstanding
activities in organic synthesis and polymer chemistry owing to
the cooperative effect between two metal centers and are
gaining an increasing amount of interest.15 Since the early
1990s, Shibasaki has conducted the synthesis of chiral
heterobimetallic lanthanide/alkali metal complexes and dis-
covered their versatile applications in a wide range of
asymmetric reactions.16 Meanwhile, several groups also
reported successful cases using heterobimetallic lanthanide/
alkali metal complexes as catalysts in catalyzing reactions such
as the reduction reaction, Strecker reaction, and Michael
reaction.17 To the best of our knowledge, the application of
RE−alkali metal complexes bearing chiral ligands in asymmetric
epoxidation of α,β-unsaturated ketones have not yet been
reported.
Our group has a long-standing interest in lanthanide complex
synthesis and their catalytic activities in organic synthesis and
polymer chemistry.18 Recently, we reported that heterobime-
tallic lanthanide−alkali metal complexes showed higher
activities in organic synthesis and ring-opening polymerization
of cyclic esters than monometallic analogues.19 These findings
motivated us to further explore the synthesis and catalytic
behavior of heterobimetallic RE−alkali metal complexes,
especially their behavior in catalyzing asymmetric reactions.
Four new heterobimetallic RE−lithium complexes stabilized by
a chiral phenoxy-functionalized prolinolate ligand were
synthesized and characterized. Furthermore, their applications
in the asymmetric epoxidations of α,β-unsaturated ketones
were also investigated, which gave rise to a series of epoxides in
good yields and up to 99% ee.
Received: July 15, 2014
© XXXX American Chemical Society
A
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Organic Letters
Letter
The in situ prepared dilithium salt LLi2 from n-BuLi and the
proligand H2L ((S)-2,4-di-tert-butyl-6-[[2-(hydroxydiphenyl-
methyl)pyrrolidin-1-yl]methyl]phenol)20 in a 2:1 ratio were
treated with a slurry of anhydrous RECl3 in THF, which gave
rise to heterobimetallic RE−lithium complexes [REL2]-
[[(THF)3Li]2(μ-Cl)] (RE = Yb (1), Y (2), Sm (3), Nd (4))
in high isolated yields (Scheme 1). The identities of complexes
1−4 were confirmed by X-ray diffraction analysis, elemental
analysis, IR spectra, as well as NMR spectroscopy in the case of
complex 2.
Table 1. Epoxidations of Chalcone Catalyzed by Complexes
1−4
a
b
c
entry cat. temp (°C) time (h)
solvent
yield (%) ee (%)
1
2
3
4
5
6
7
8
9
1
2
3
4
1
1
1
1
1
1
1
1
1
25
25
25
25
25
25
25
25
0
4
4
THF
94
93
95
93
94
95
88
88
74
66
70
50
70
trace
51
50
15
<1
39
38
9
THF
4
THF
4
THF
d
e
f
4
THF
Scheme 1. Synthesis of Complexes 1−4
4
THF
4
THF
4
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
73
89
85
85
95
94
nd
12
12
12
12
20
20
10
11
12
13
14
−10
0
g
h
h
i
0
0
0
a
Reactions were performed with chalcone (0.5 mmol), TBHP (0.6
b
mmol), and MS 4 Å (100 mg) in 3.3 mL of solvent. HPLC yields.
c
d
e
Determined by chiral HPLC. 1.5 equiv of oxidant. 2.0 equiv of
f
g
oxidant. The oxidant is CMHP. Addition of 30 mol % of Ph3PO as
h
i
the additive. The amount of MS 4 Å is 15 mg (30 mg/mmol). The
control reaction without any metal, just using 20 mol % of ligand. nd =
not detected.
The molecular structures of complexes 1−4 are iso-
morphism, which comprise a rare discrete [[(THF)3Li]2(μ-
Cl)]+ cation and a [L2Ln]− anion.21 The solid-state structure of
complex 1 is depicted in Figure 1 as a representative.22
considered critical in catalyzing the transformation and
controlling the enantioselectivity.14e,16k,23
Using ytterbium−lithium complex 1 as the catalyst, the
reaction conditions, including the amount of oxidant and
molecular sieves, temperature, reaction time, and solvent, were
screened (Table 1). Use of 1.2 equiv of oxidant TBHP proved
to be efficient (Table 1, entries 1, 5, and 6), while CMHP led to
negligible enantioselectivity (Table 1, entry 7). The reaction is
sensitive to solvent, and acetonitrile proved to be the optimal
choice (Table 1, entries 1 and 8; Table S10, Supporting
Information). When the reaction temperature dropped to 0 °C,
the ee value was improved to 89%, although the yield was
sacrificed (Table 1, entries 8 and 9). Further decreasing the
temperature to −10 °C resulted in similar ee values and even
lower yield (Table 1, entry 10). Moreover, the addition of
Ph3PO failed to improve the ee value of the product as
expected (Table 1, entry 11). In view of both yield and ee value
of the product, the optimal conditions of 20 h reaction time in
CH3CN at 0 °C with the MS 4 Å loading of 30 mg/mmol was
selected, which yielded the epoxidation product in moderate
yield and excellent enantioselectivity (Table 1, entry 13).
To gain a deep insight into the substrate scope of the
asymmetric epoxidation reaction catalyzed by the RE−lithium
complex, a series of reactions with readily accessible chalcone
derivatives were carried out. To our delight, a number of
substrates underwent the asymmetric reactions smoothly to
produce the corresponding epoxides in good to excellent
enantiomeric excess of greater than 99% and moderate yields of
60−79%. The results are listed in Scheme 2.
Figure 1. Solid-state structure of complex 1.
To explore the catalytic behavior of ionic heterobimetallic
RE−lithium complexes 1−4, the asymmetric epoxidation of
α,β-unsaturated ketones was investigated. Using commercially
available chalcone as the starting material, asymmetric
epoxidation reaction was catalyzed by complexes 1−4 in the
presence of oxidant TBHP (tert-butyl hydroperoxide) or
CMHP (cumene hydroperoxide). The results of this model
reaction are summarized in Table 1.
To our delight, all heterobimetallic complexes 1−4 catalyzed
this transformation to yield the targeted epoxide in 93−95%
yields with 10 mol % catalyst loading, while the enantiose-
lectivities are far apart (Table 1, entries 1−4). The central metal
significantly influences the enantiomeric excess of the product
with the decreasing tendency of Yb(1) ∼ Y(2) > Sm(3) >
Nd(4), which is opposite to the trend of the ionic radii.
Presumably, the congestion around the RE, the Lewis acidity of
the RE, and the nucleophilicity of the RE peroxide species are
in a good compromise for complexes 1 and 2, which is
The electronic effect of the substituents on the phenyl ring
significantly affects the reaction outcomes. Substrates bearing
electron-donating groups at the para position of either phenyl
ring gave rise to corresponding epoxides in excellent ee values
of 94−99% and 65−72% yields (Scheme 2, 6b−d and 6n−p).
When the electron-donating groups are located at the ortho
B
dx.doi.org/10.1021/ol5020398 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
asymmetric epoxidation of α,β-unsaturated ketones but
substantially different enantioselectivity. Yb−Li complex 1
proved to be the optimal catalyst in terms of both efficiency
and selectivity, and a wide range of chalcones of different
electronic and steric properties were expediently converted to
various epoxides in good to excellent enantiomeric excess and
moderate yields at 0 °C in the presence of 10 mol % of catalyst.
Scheme 2. Epoxidation of α,β-Unsaturated Ketones
Catalyzed by Complex 1
a
ASSOCIATED CONTENT
* Supporting Information
■
S
General procedures for preparation of complexes and substrates
1
and catalysis, characterization data, H and 13C NMR spectra,
HPLC chromatograms, crystallographic data for complexes 1−
4 [CCDC: 980573 (1), 980574 (2), 980575 (3), 980576 (4)]
and figures depicting solid-state structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zhaobei@suda.edu.cn.
*E-mail: yaoym@suda.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully thank Prof. Xingwang Wang (Soochow
University) for his kind help. We also gratefully acknowledge
financial support from the National Natural Science Founda-
tion of China (Grant Nos. 21172165, 21372172 and
21132002), PAPD, and the Qing Lan Project.
a
Reactions were performed with chalcone and its derivatives (0.5
mmol), TBHP (0.6 mmol), and MS 4 Å 15 mg in 3.3 mL of solvent at
0 °C for 20 h in the presence of 10 mol % of complex 1. HPLC yields.
The ee values were determined by chiral HPLC.
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